Oilfield production systems normally feature controls and monitoring devices in the well, at the Christmas tree and at the hydrocarbon receiving facility.
There is a need for communication between the downhole devices, such as sensors, actuators, etc, and the other parts of the oilfield production system. The communication between the downhole devices and access points outside the Christmas tree is today normally performed via cabling and piping in the annulus between the production tubing and the casing. At the Christmas tree, the production tubing is hung off and the tubing hanger contains electric, optical and hydraulic/chemical couplers between the tubing hanger and the Christmas tree. In the Christmas tree, communication is via cables, optical fibers and piping to new connectors at the access points external at the Christmas tree. From these access points in a subsea system, the communication to the overall control system typically to a Subsea Control Module (SCM) via traditional cables, optical fibers, hoses and piping. This downhole communication system is robust, does not interfere with the internal production bore and may contain many passages, but it may not be changed or repaired during the life of field without expensive and complex operations. These operations involve retrieval of the Christmas tree and/or the production tubing.
Plugging devices are used in oilfield production systems in the production tubing during the installation phase to provide temporary isolation and sealing, and during the production phase to provide more permanent isolation and sealing of the temporary well access.
Several plugging devices for oil- and gas wells are known. Such plugging devices are for example used in a horizontal Christmas tree, as illustrated in FIG. 1. One such plugging device is known from Norwegian patent 322829.
WO9963202 describes a method and arrangement for monitoring reservoir parameters in subsea wells.
U.S. Pat. No. 6,478,086 describes a method for plugging wells, where communication is provided between a sensor provided under a seal down in the well and a connection part. Over the seal the well is filled with a temperable, liquid sealant.
The object of the invention is to provide a subsea horizontal Christmas tree comprising a plugging device comprising communication means for downhole communication, which provides possibilities for permanent well access for monitoring and control in horizontal trees. Moreover, the invention provides the possibility for easy retrofit of new downhole sensors, actuators and tools.